1. Field of the Invention
The present invention relates to the application of a mud comprising scleroglucan to horizontal or directional boreholes and in particular to boreholes having a deviation angle with respect to the vertical of between 40.degree. and 70.degree..
2. Description of the Related Art
The drilling mud is a more or less complex mixture of a base liquid, water or oil, and various products, used for drilling wells. This mud, injected in the drill-pipe string, circulates by a rising movement in the annular space contained between the walls of the geological formations drilled and the pipe string.
In the case of vertical drilling, the flow of the mud in the annular space is most often of the laminar type. This flow ensures removal of the cuttings dislodged at the working face which are transported toward the surface in the annular space and removed at the surface. Bailing-up of the drilling bit is thus avoided.
In laminar flow, a particle situated at the centre of the annular space has a velocity greater than the average velocity and a velocity less than the average velocity in the vicinity of the walls of the formations or of the pipe string. A particle transported to a point of low velocity will have a tendency to topple or to stay there. If the flow rate is too low, every particle will have a tendency to go back down because of gravity. In order to avoid this phenomenon and to ensure good cleaning of the drilling well, it is most often sufficient in vertical drilling to increase the viscosity of the mud so that its carrying capacity is such that the drill-cuttings can be held in quasi-suspension in the mud when the flow rate of the latter is zero.
In the case of deviated wells, however, these phenomena of falling back under gravity are more difficult to avoid and most often veritable beds of cuttings of variable thickness which are formed along the lower generatrix of the well are observed. These accumulations can be particularly big in the region of changes in the degree of inclination of the well, in the curves of the drilling, and can in consequence reduce the quality of the cleaning of the well and cause increased problems of friction and thus decrease the rate of penetration of the well, or indeed cause blocking of the drilling. In the case of drilling with a water-based mud, the solution usually employed for freeing and removing or reducing the accumulations of drill-cuttings during drilling of deviated wells consists most often in causing a change of the flow conditions. For this purpose, for example fixed volumes of a mud having a viscosity considerably less than or considerably greater than that used under normal conditions are periodically injected into the well in order thus to cause either turbulent conditions characterised by equal velocities throughout the annular space, or plug conditions characterised by velocities which are practically constant and of the same direction throughout the annular space. In practice, however, this solution turns out to be tricky to implement and the results obtained are highly random.
Another solution to limit the problem of accumulation of drill-cuttings during drilling of deviated wells consists in employing as the drilling mud not a water-based mud but an oil-based mud.
Oil-based muds are characterised by a Bingham-type rheological behaviour with the existence of a viscosity even at very small shears. The muds retain this characteristic in conditions at the bottom of a well and at high temperature. Most of the time, the muds consist of an emulsion of water in 30 to 40 % by volume of oil. If the viscosity of the emulsion is not sufficient, it is adjusted to the desired value by the addition of inorganic colloids such as bentonite for example.
It is noticed that, with the use of oil-based muds, accumulations of drill-cuttings and problems connected with these accumulations are less critical than with water-based drilling. Oil-based muds furthermore have a much higher lubricating power than water-based muds.
These muds however are not only expensive but can be tricky to employ, in particular in offshore drilling, but above all they are extremely polluting to the point that the legislation of some countries forbids their disposal. The muds and the cuttings which they carry up must therefore increasingly be processed at the surface in order to avoid any pollution.
This is why in drilling oil-based muds are used only in a very limited number of cases.
Furthermore, the introduction of solids in order to adjust the viscosity amongst other things increases the risks of clogging and complicates the problems of removal at the surface.